The present invention relates to the field of communications systems, and more particularly to a system and method for selective filter isolation in an asymmetric digital subscriber line application.
Asymmetric digital subscriber line (ADSL) technology is a communications format designed to facilitate large downstream bandwidth (e.g. from the carrier to the customer), while leveraging the existing base of twisted pair telephone lines. ADSL provides a significant advantage over other digital subscriber line technologies in its ability to facilitate simultaneous transmission of analog voice telephony and high-speed digital transmission. In addition, by using the large existing base of twisted pair telephone lines, ADSL provides marked cost savings over other high-speed transmission alternatives, such as cable modems and fiber to the neighborhood, which require placement of new high-speed cabling for their implementation.
Full rate ADSL, sometimes referred to as xe2x80x9cADSL heavy,xe2x80x9d utilizes a splitter (which acts as a filter) at the customer""s premises to separate the higher frequency DSL data from the lower frequency voice signals. Throughout this document, the terms xe2x80x9clow frequencyxe2x80x9d and xe2x80x9chigh frequencyxe2x80x9d are used to denote relative frequencies, and are not intended to correspond to any particular frequency range.
Using a splitter at the customer""s premises protects the DSL data from interference associated with lower frequency voice line transients caused, for example when a telephone rings. A disadvantage associated with full rate ADSL is that its installation generally requires a technician to actually go to each customer""s premises (xe2x80x9croll a truckxe2x80x9d) and install a splitter at the customer""s network interface device.
In an effort to avoid the expense of requiring a technician to install a splitter at each customer""s network interface device, the industry has begun development of a lower-cost, less complex version of full ADSL, known as xe2x80x9cADSL Lite.xe2x80x9d The theory behind ADSL Lite is that by eliminating the need for a splitter at the network interface device, ADSL Lite eliminates the cost of sending a technician to activate the service.
In actual implementation, due to various factors, such as large variations in impedances between various manufacturers"" consumer telephone equipment, both full ADSL and ADSL Lite have suffered from interference between analog voice traffic and higher frequency DSL signals. One approach to eliminating this interference is to permanently place a low pass filter in series with each telephone at the customer""s premises. While this approach eliminates interference between lower frequency voice traffic and higher frequency data traffic, it has created problems of its own. For example, where a customer""s premises has more than one telephone, the unterminated filters associated with on-hook telephones create resonant circuits that result in interference in the circuit of the off-hook telephone. This interference not only hinders the performance of the telephone equipment, but may also create irregularities and/or errors in the higher frequency ADSL data communicated to and from the customer""s premises.
In accordance with the present invention, in a system including a plurality of telephones coupled to an asymmetric digital subscriber line, a circuit operable to facilitate communication using the asymmetric digital subscriber line comprises a filter having a corner frequency and being operable to be coupled between one of the plurality of telephones and the asymmetric digital subscriber line, and to attenuate transients associated with the telephone operating in an off-hook state. The circuit further comprises a switch coupled to the filter and operable to effect a change in the corner frequency of the associated filter depending on whether the telephone coupled to the filter is operating in an on-hook or an off-hook state, the change in the corner frequency operating to attenuate parasitic interference associated with telephones in an on-hook state.
Technical advantages of the present invention include a system and method for facilitating communications using an asymmetric digital subscriber line that reduces or eliminates low-frequency transients interfering with higher frequency ADSL data, while also reducing or eliminating parasitic interference associated with other on-hook telephones coupled to the asymmetric digital subscriber line. For example, the invention can be designed so that a telephone going on-hook causes the inductance of the unterminated filter coupled to that telephone to increase, thereby reducing the corner frequency of the unterminated filter to a point where parasitic interference associated with that unterminated filter does not affect other off-hook telephones in the system. As a further example, the invention can be designed so that a telephone going off-hook causes an increase in the capacitance of the terminated filter coupled to that telephone, lowering the corner frequency of the terminated filter to a point where the filter operates to attenuate parasitic interference associated with other on-hook telephones in the system. Other technical advantages are readily apparent to one of skill in the art from the attached figures, description, and claims.